Cellular communications systems are commonly employed to provide voice and/or data communications to a plurality of subscribers within an array of defined geographical regions referred to as cells. Examples of voice applications include analog cellular radiotelephone systems such as AMPS, ETACS, NMT-450, and NMT-900, as well as digital cellular radiotelephone systems such as IS-54B in North America and GSM in Europe. These systems, and others, are described, for example, in the book titled Cellular Radio Systems by Balston, et al., published by Artech House, Norwood, Mass., 1993. Other types of cellular systems include data communications systems such as CDPD, a digital data transmission system designed to communicate data packets over an AMPS cellular infrastructure, and personal Air Communications Technology (pACT), a two-way paging and messaging protocol and system specification for narrow-band personal communications systems (PCS), a packet data system based on the widely used Internet Protocol (IP).
FIG. 1 illustrates a typical terrestrial cellular radiotelephone communication system 20 as in the prior art. The cellular radiotelephone system may include one or more subscriber units 21, communicating with a plurality of cells 36 served by base stations 23 and a switching office, here shown as a Mobile Telephone Switching Office (MTSO) 25. Although only three cells 36 are shown in FIG. 1, a typical cellular network may include hundreds of cells, may include several switching offices, and may serve thousands, if not millions, of subscriber units. The cells 36 generally serve as nodes in the communication system 20, from which links are established between radiotelephones 21 and the MTSO 25, by way of the base stations 23 serving the cells 36. Thus, for example, a duplex communication link 32 may be effected between two subscriber units 21 or between a subscriber unit 21 and an end user connected to the cellular network 20, for example, an end user 33 utilizing a conventional landline telephone connection through a public telephone switched network 30. The base station 23 functions to relay communications between the cell and the mobile station 21. Those skilled in the art will appreciate that similar cellular structures may be implemented using satellites or similar devices serving similar functions to the terrestrial base stations 23 illustrated in FIG. 1.
FIG. 2 illustrates a typical cellular packet data communications system, specifically, an architecture typically employed in CDPD and pACT networks. An important element of the network is the pACT Data Intermediate System (PDIS) 210, a specialized router that allows a subscriber unit 220 to move among cells 240 without losing connectivity within a fixed infrastructure network such as, for example, an internet. In particular, the PDIS 210 typically provides mobility management by constructing and continuously updating a directory/routing table that maps subscriber unit IP addresses to a temporary address and channel stream that identifies which base station 230 the subscriber unit 220 is currently connected. Typically, communication between a PDIS and a subscriber unit is transparent to a Fixed End Subscriber (FES) 250 communicating with the subscriber unit 220. The FES 250 will issue an IP packet that is routed via an internet router 260 to the subscriber unit's home PDIS. The home PDIS may either route this packet to the subscriber unit via a specific base station 230 associated with the home PDIS or pass the packet to a different PDIS if the subscriber unit 220 has moved into a new radio coverage area served by the second PDIS, with the new PDIS taking on responsibility for routing the packet to the correct base station 230.
One way in which the PDIS can track the location of a subscriber unit is to require the subscriber unit to announce its transfers between base stations by sending a location update message on the reverse or uplink, i.e., subscriber unit to base station, channel after a cell transfer has been performed. In systems with limited reverse channel bandwidth, however, a significant portion of the reverse channel may be consumed by the transmission of location updates by subscriber units. This typically results in a congested reverse channel and a decrease in network throughput. Also, mobiles with limited battery capacity sacrifice significant battery power to transmit their location updates as they traverse each cell boundary.